THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP99/05245.
The present invention relates to a reflection-type liquid crystal display device.
With the rapid spread of data communications apparatuses such as cellular phones, PHS""s, PDA""s (personal digital assistants), and the like, infrastructure has been coming to be in good order for anyone to get access and communicate readily at anytime and anywhere. Since these apparatuses are designed for use with mobility, which require display devices of light weight, thin, and low power consumption, liquid crystal display devices have become the dominating components at present. Such a liquid crystal display device provides a display with liquid crystal molecules that change transmission factor of light when being driven by several volts of effective voltage. However, since the liquid crystal is non-luminescent material, it requires another form of light source. Such a light source needs to be supplied with very large electric power as compared to electric power for driving the liquid crystal. On the other hand, a reflection-type liquid crystal display device (hereinafter referred to as xe2x80x9creflection-type LCDxe2x80x9d) takes extremely low power consumption, and realizes a display device that makes the best use of a feature inherent to the liquid crystal, as it is provided with a reflection plate underside of it to help display by using surrounding light. The reflection-type LCD is coming to be indispensable as a kind of displays for the personal digital assistants and the like.
However, the reflection-type LCD of the prior art has had a problem of not being capable of providing sufficient brightness in all directions other than a direction of the specular reflection of incident light to the display device, because it uses the surrounding light to display. For this reason, there has been suggested a structure in that a scattering film or the like is used to diffuse light toward other directions than that of the specular reflection in order to expand viewing angle.
The one described in Japanese Patent Laid-open Publication H08-201802 (hereinafter referred to as xe2x80x9cprior art structure Axe2x80x9d), and another having a scattering feature provided by asperities formed on an upper substrate, etc. described in Japanese Patent Laid-open Publication H08-338993 (hereinafter referred to as xe2x80x9cprior art structure Bxe2x80x9d), for examples, have been known as some of the reflection-type LCD""s of the prior art. FIG. 5 shows a configuration of the reflection-type LCD having the prior art structure A.
In the above-described prior art structure A and the prior art structure B, there still exist various problems depending on scattering characteristic of a scattering film used for each configuration.
If the scattering characteristic is weak, i.e. low in diffuse reflection factor, there is hardly any light reflected toward a front direction (direction normal to the LCD surface) of the reflection-type LCD of the prior art structure A or the prior art structure B, since the light incident upon it is scarcely scattered, but reflected toward a direction of the specular reflection, thereby giving rise to a problem that it displays very dark white.
On the other hand, if the scattering characteristic is intense, or the diffuse reflection factor is high, there occurs backward scattering upon incidence of the surrounding light on the reflection-type LCD of the prior art structure B, which is provided with the scattering characteristic by asperities of its substrate. Since this prevents the reflection factor from being sufficiently low in a black display, it also gives rise to another problem of not providing a high contrast. Furthermore, apart from the case of the prior art structure B, in the case of a configuration in the prior art structure A, which uses a front scattering film, multiple scattering can occur within the scattering film when light passes through it, because the scattering characteristic is intense. This changes a polarizing state of the transmitting light at its entry into the film as opposed to outgoing from the film. This results in a failure to prove a good black display with a low reflection factor, and thereby giving rise to a problem of not providing a high contrast.
Moreover, if the scattering characteristic is excessively intense in the above-said prior art structure A and the prior art structure B, the light diffused upon departing from the scattering film or a scattering layer formed with asperities gives rise to still another problem causing an image to appear blurred.
A reflection-type LCD of another structure has been suggested, in that a scattering layer is arranged between a polarizing film and a liquid crystal layer, as means to solve the above problems. However, even the above-described structure could not solve the problems for some devices while it could for the others, so that there had been a desire for detailed structural conditions to be established so as to solve the above problems reliably.
The present invention pertains to a reflection-type LCD comprising a liquid crystal cell, a polarizing film, a birefringent film, a scatteringlayer, and a light reflection means, wherein the scattering layer has a diffuse reflection factor xe2x80x9cRxe2x80x9d of 7% or greater but 110% or less when an incident angle is in the range of 25xc2x0 or greater and 45xc2x0 or less.
Provided that the diffuse reflection factor xe2x80x9cRxe2x80x9d used here is defined by the following formula:
R=R1/R2xc3x97100
The R1 and R2 are defined as follows. Prepared first is an aluminum regular reflection plate, of which a reflection factor in a direction of the normal line is 91% to incident light of 546 nm in wave length having an incident angle (an angle with respect to the normal line of an object) of 0xc2x0. A reflection factor to a direction of the normal line is assumed to be R1, when light is irradiated at an incident angle of 0xc2x0 to the object scattering layer adhered onto the aluminum regular reflection plate. Also, a reflection factor to a direction of the normal line with respect to an MgO layer having a thickness equal to that of the object adhered to the aluminum regular reflection plate is assumed to be R2, when light is irradiated to the MgO layer at an incident angle of 25xc2x0.
By adopting the structure as set forth above, there can be provided the reflection-type LCD capable of realizing excellent black and white display, thereby producing a high contrast and a clear image without blur of the image.
Further, the present invention is characterized by the scattering layer of which the diffuse reflection factor xe2x80x9cRxe2x80x9d is 60% or greater at the incident angle of 25xc2x0.
In addition, the scattering layer of this invention has a total light transmission factor of 80% or greater for incident light from a direction of the normal line to the scatteringlayer. According to this structure, there can be produced not only good white display with a higher reflection factor, but also excellent black display with a low reflection factor, because of a reduced backward scattering, thereby realizing an image of improved visibility.
Furthermore, the scattering layer of this invention is characterized by being isotropic in the scattering characteristic. With this structure, there can be realized an excellent black and white display in a front direction of the reflection-type LCD without dependent upon a direction of light incident on the LCD, thereby achieving a high contrast and a clear image without blur of the image.
Moreover, the scattering layer of this invention is characterized by further having adhesiveness. By adopting this structure, there can be provided with scattering characteristic in the adhesive portion used for bonding the reflection-type LCD. This can reduce an overall thickness of the reflection-type LCD as compared with the one employing a scattering layer formed separately in a film configuration. In addition, by adopting the adhesive scattering layer formed between a liquid crystal cell and a birefringent film, the adhesive layer is not separately needed. This can shorten a distance between the scattering layer and a reflecting surface, and thereby the blur of image can be reduced.